It is a well-known technique that wire electric discharge machining can be performed at high speed while preventing breaking of wire by applying large and small current pulses to an inter-electrode portion according to a state of the inter-electrode portion represented by a length of a no-load time. On the other hand, a repetition frequency in wire electric discharge machining is about 60 kilohertz to 100 kilohertz. However, when two kinds of large and small current pulses are applied, a ratio of occurrence of the respective current pulses are 50 percents for the large current pulse and about 50 percents for the small current pulse, and the repetition frequency of the large current pulse is 30 to 50 kilohertz.
In general, when current peaks of respective large and small current pulses are changed according to a length of a no-load voltage of a preliminary discharge pulse, if control is performed with a small current pulse (short circuit, immediate discharge) within a no-load time of 2 microseconds from voltage application and control is performed with a large current pulse (normal discharge) in a no-load time of 2 microseconds or more from voltage application, a ratio of occurrence of normal discharge is a value about ½ to ⅓ of a total pulse number. In addition, in short circuit or immediate discharge, occurrence of sludge tends to be time concentrated or position concentrated due to influence of flow of machining liquid or the like, and when occurrence of sludge is discharge concentrated, a repetition frequency increases.
FIG. 1 is a diagram showing a list of characteristic data concerning switching response times of switching elements with different rated capacities. In FIG. 1, three field effect transistors (FETs) 1 to 3, one insulated gate bipolar transistor (IGBT), and one IGBT module are shown as switching elements, and a “capacity”, a “gate input capacity”, a “turn-on time”, a “turn-off time”, and a “minimum pulse width” are indicated for the respective switching elements.
The FET 1 has the “capacity” of 500 volts and 3 amperes, the “gate input capacity” of 330 picofarads, the “turn-on time” of 25 nanoseconds, the “turn-off time” of 50 nanoseconds, and the “minimum pulse width” of 77 nanoseconds. The FET 2 has the “capacity” of 500 volts and 10 amperes, the “gate input capacity” of 1050 picofarads, the “turn-on time” of 85 nanoseconds, the “turn-off time” of 135 nanoseconds, and the “minimum pulse width” of 210 nanoseconds. The FET 3 has the “capacity” of 500 volts and 30 amperes, the “gate input capacity” of 2800 picofarads, the “turn-on time” of 172 nanoseconds, the “turn-off time” of 300 nanoseconds, and the “minimum pulse width” of 472 nanoseconds.
The IGBT has the “capacity” of 600 volts and 75 amperes, the “gate input capacity” of 4100 picofarads, the “turn-on time” of 600 nanoseconds, the “turn-off time” of 800 nanoseconds, and the “minimum pulse width” of 1400 nanoseconds. The IGBT modules has the “capacity” of 600 volts and 400 amperes, the “gate input capacity” of 20000 picofarads, the “turn-on time” of 700 nanoseconds, the “turn-off time” of 1100 nanoseconds, and the “minimum pulse width” of 1800 nanoseconds.
In general, a switching element tends to have longer switching response time when capacities of rated voltage and rate current are larger. In addition, as shown in FIG. 1, in general, a switching element having a smaller current capacity tends to have a smaller gate input capacity even if a rated voltage is the same. In other words, since smaller electric power is required for driving, an operation of the switching element can be performed at higher speed.
Since an element having a small ON resistance and small heat generation among the switching elements has a large gate input capacity, the switching element cannot perform a high-speed operation. In addition, since an element having a small gate input capacity and capable of performing a high-speed operation has a large ON resistance and a small maximum current capacity and also has a large heat value, there is a problem in that, for example, cooling of the element is costly and the element occupies a large space.
FIG. 2 is a circuit diagram showing an example of a structure of a conventional power supply device for electric discharge machining. FIG. 3 is a block diagram showing a structure of a power supply control circuit that controls to drive switching elements S11a and S11b of a power supply unit for machining discharge 101.
In FIG. 2, an electrode E consisting of a wire and a workpiece W serving as the other electrode are arranged to be opposed to each other at an appropriate interval in an electric discharge machining unit 100. A power supply unit for machining discharge 101 and a power supply unit for preliminary discharge 102 are provided with respect to this electric discharge machining unit 100.
The power supply unit for machining discharge 101 includes a variable DC power supply V11, switching elements (e.g., FETs) S11a and S11b, and diodes D11, D12, D13, and D14. The power supply unit for preliminary discharge 102 includes a variable DC power supply V21, a switching element (e.g., FET) S21, a resistor R21, and diodes D21 and D22.
In the power supply unit for machining discharge 101, a cathode electrode of the diode D11 and a source electrode of the switching element S11a are connected at a positive electrode terminal of the DC power supply V11. In addition, a source electrode of the switching element S11b and an anode electrode of the diode D12 are connected at a negative electrode terminal of the DC power supply V11.
A drain electrode of the switching element S11a is connected to a cathode electrode of the diode D12 and an anode electrode of the diode D13, and a cathode electrode of the diode D13 is connected to the workpiece W. A floating inductance L11 is present in a connection line for the cathode electrode of the diode D13 and the workpiece W.
A drain electrode of the switching element S1b is connected to an anode electrode of the diode D11 and a cathode electrode of the diode D14. A floating inductance L12 is present in a connection line for an anode electrode of the diode D14 and the electrode E.
In the power supply unit for preliminary discharge 102, a source electrode of a switching element S21 is connected to a positive electrode terminal of the DC power supply V21, and a drain electrode of the switching element S21 is connected to an anode electrode of the diode D21. A cathode electrode of the diode D21 is connected to the workpiece W. A floating inductance L21 is present in a connection line for the cathode electrode of the diode D21 and the workpiece W.
A cathode electrode of the diode D22 is connected to a negative electrode terminal of the DC power supply V21, and an anode electrode of the diode D22 is connected to the electrode E. A floating inductance L22 is present in a connection line for the anode electrode of the diode D22 and the electrode E. A stray capacitance C11 is present between the connection line for the cathode electrode of the diode D21 and the workpiece W and the connection line for the anode electrode of the diode D22 and the electrode E.
As shown in FIG. 3, the power supply control circuit, which controls to drive the switching elements S11a and S11b of the power supply unit for machining discharge 101, includes a discharge detecting circuit 13 that detects a discharge current flowing to an inter-electrode portion (W-E) 105, which is a part between the electrode E and the workpiece W, an oscillation control circuit 14 that receives a start instruction pulse signal PK from the discharge detecting circuit 13, and drive circuits 15a and 15b to which a control pulse signal PC is inputted in parallel from the oscillation control circuit 14. The switching element S11a and S11b are adapted to receive a drive pulse signal PD from the drive circuits 15a and 15b and apply a machining pulse signal PS to the inter-electrode portion (W-E) 105.
Next, operations of the conventional power supply device for electric discharge machining will be explained with reference to FIGS. 2 to 4. Note that FIG. 4 is a diagram explaining a principle of operation of the conventional power supply device for electric discharge machining shown in FIG. 1.
First, meaning of reference signs shown in FIGS. 2 and 3 will be explained. In FIG. 2, a current IWE10 flowing from the stray capacitance C11 to the electric discharge machining unit 100 is a discharge start current. A current IWE11 flowing from the power supply unit for machining discharge 101 to the electric discharge machining unit 100 is an electric discharge machining current. A current IWE22 flowing from the power supply unit for preliminary discharge 102 to the electric discharge machining unit 100 is a discharge maintenance current. A current IWE flowing from the workpiece W to the electrode E is an inter-electrode current. In addition VWE denotes an inter-electrode voltage.
In FIG. 3, reference signs tk, tc, td, ts denote times (delay times) required for processing for receiving inputs in the respective circuits and generating and outputting desired signals, respectively, and tr denotes a delay time obtained by summing up the delay times. In other words, the delay time tr is a time from the time when the discharge detecting circuit 13 detects occurrence of discharge in the inter-electrode portion (W-E) 105 of the electric discharge machining unit 100 until the time when the switching elements S11a and S11b can apply the machining pulse PS to the inter-electrode portion (W-E) 105. Note that the inter-electrode portion (W-E) 105 will be hereinafter simply represented as an inter-electrode portion.
Incidentally, in FIGS. 2 and 3, when the switching element S21 of the power supply unit for preliminary discharge 100 is turned ON in a state in which the inter-electrode portion between the electrode E and the workpiece W is not discharging and is not short-circuited, a voltage at the DC power supply V21 appears in the inter-electrode portion. At the same time, the stray capacitance C11 in the circuit is charged to the voltage at the DC power supply V21. Note that a distance between the electrode E and the workpiece W is controlled by a numerical control device and a servo drive control device, which are not shown in the figures, such that discharge occurs.
When discharge occurs in the inter-electrode portion due to an output voltage at the DC power supply V21, first, charges stored in the stray capacitance C1 in the circuit are capacitor-discharged to the inter-electrode portion, and the discharge start current IWE10 flows. Consequently, a conductive path is formed in the inter-electrode portion. To maintain this conductive path, since a current has to be caused to continue to flow to the inter-electrode portion even after the charges in the stray capacitance C11 in the circuit are fully discharged, the switching element S21 is kept ON.
As a result, the discharge maintenance current IWE22 flows in a path of the DC power supply V21→the switching element S21→the resistor R21→the diode D21→the floating inductance L21 in the circuit→the workpiece W→the electrode E→the floating inductance L22 in the circuit→the diode D22→the DC power supply V21, and the conductive path formed in the inter-electrode portion is maintained. At this point, since the discharge maintenance current IWE22 flows through the resistor R21, a maximum value of the discharge maintenance current IWE22 is limited to IWE22(max)=V21/R21 by the resistor R21. Therefore, since this discharge maintenance current IWE22 has a relatively small current value and is weak as machining energy, the discharge maintenance current IWE22 has a role of a preliminary discharge current for causing the large electric discharge machining current IWE11 to flow. As described below, occurrence of discharge is detected according to the discharge maintenance current IWE22 appearing in the inter-electrode portion simultaneously with the occurrence of discharge, and the large electric discharge machining current IWE11, which is to be caused to flow to the inter-electrode portion, is outputted to the inter-electrode portion with a delay of a time tr from time t0 when the occurrence of discharge is detected.
In other words, the discharge detecting circuit 13 detects the drop of an inter-electrode voltage due to the occurrence of discharge in the inter-electrode portion and outputs the start instruction pulse signal PK for a large current output to an oscillation control circuit 14. The oscillation control circuit 14 outputs the control pulse signal PC of a pulse width, which is set according to a machining state in the inter-electrode portion, to the drive circuits 15a and 15b. At the same time, the drive circuit 15a drives to turn ON the switching element S11b according to the drive pulse signal PD in the same manner.
Here, when all the switching elements S11a, S11b, and S21 come into an ON operation state, a circuit, to which plural DC power supplies with different voltages are connected, is formed. In this case, elements in the circuit are likely to be destroyed due to a potential difference including a surge voltage. Thus, when the switching elements S11a and S11b are turned ON, the switching element S21 is turned ON as a safety measure.
In the power supply unit for machining discharge 101, the switching elements S11a and S11b perform an ON operation simultaneously, whereby the large electric discharge machining current IWE11 flows in a path of the DC power supply V11→the switching element S11a→the diode D13→the floating inductance L11 in the circuit→the workpiece W→the electrode E→the floating inductance L12 in the circuit→the diode D14→the switching element S11b→the DC power supply V11.
When the control pulse signal PC from the oscillation control circuit 14 disappears, the drive circuits 15a and 15b drive to turn OFF the switching elements S11a and S11b, respectively. At this point, the electric discharge machining current IWE11 is to be caused to continue to flow in the circuit by an inductive action of the floating inductances L11 and L12 in the circuit. As a result, the electric discharge machining current IWE11 returns to the DC power supply V11 in a path of the floating inductance L11 in the circuit→the workpiece W→the electrode E→the floating inductance L12 in the circuit→the diode D14→the diode D11→the DC power supply V11 and is regenerated.
Next, in FIG. 4, the switching element S21 (3) performs an ON operation, whereby an inter-electrode voltage VWE (1) changes to a certain voltage (voltage at the DC power supply V21), and the capacitor C11 is charged. When a discharge start current IWE10 (2) caused by discharge of the capacitor C11 starts flowing at the discharge start time t0, the inter-electrode voltage VWE (1) starts dropping. In addition, a discharge maintenance current IWE22 (4) starts flowing with a rising inclination of V21/(L21+L22) affected by influence of the floating inductance L21 and L22.
The inter-electrode voltage VWE (1) reaches a lowest discharge voltage Va at certain time after the time tk has elapsed from the discharge start time t0 and maintains the discharge voltage Va after that. The discharge maintenance current IWE22 (4) reaches a predetermined value (IWE22(max)=V21/R21) around time when the discharge start current IWE10 (2) passes a peak value (certain time after the time tk has elapsed from the discharge start time t0). Then, when the time tr has elapsed from the discharge start time t0, since a switching element S11 (8) serving as the switching elements S11a and S11b performs an ON operation, the switching element S21 (3) maintains an ON operation state until the time tr elapses. Therefore, the discharge maintenance current IWE2 (4) maintains the predetermined value (IWE22(max)=V21/R21) in the time tr during which the switching element S21 (3) is performing the ON operation.
When the time tk has elapsed from the discharge start time t0, the discharge detecting circuit 13 detects drop of the inter-electrode voltage VWE (1) to a predetermined value or less and generates a start instruction pulse signal PK (5). This start instruction pulse signal PK (5) is outputted in a time largely exceeding the time tr in which the switching element S21 (3) is performing the ON operation. Subsequently, when a time (tk+tc) has elapsed from the discharge start time t0, the oscillation control circuit 14 generates a control pulse signal PC (6). This control pulse signal PC (6) is outputted in a time (td+ts).
Subsequently, when a time (tk+tc+td) has elapsed from the discharge start time t0, the drive circuits 15a and 15b generate a drive pulse signal PD (7). A generation period of this drive pulse signal PD (7) is the same as a generation period of the control pulse signal PC (6). Finally, when a time (tk+tc+td+ts) has elapsed from the discharge start time t0, that is, when the time tr has elapsed from the discharge start time t0, a switching element S11 (8) serving as the switching elements S11a and S11b performs an ON operation, and a machining pulse signal PS is outputted. A period in which the switching element S11 (8) performs the ON operation is the same as the generation period of the drive pulse signal PD (7).
When the switching element S11 (8) performs the ON operation, an electric discharge machining current IWE11 (9) starts flowing. Since the electric discharge machining current IWE11 (9) flows through the floating inductances L11 and L12, the electric discharge machining current IWE11 (9) continues to rise with an inclination of V11/(L11+L12) in a period in which the switching element S11 (8) is in an ON operation state. Usually, since a voltage at the DC power supply V11 is two to three times as high as a voltage at the DC power supply V21, a rising inclination of the electric discharge machining current IWE11 (9) is steeper than a rising inclination of the discharge maintenance current IWE22 (4). When the switching element S11 (8) performs an OFF operation, the electric discharge machining current IWE11 (9) turns to drop.
Eventually, the inter-electrode current IWE (10) changes to IWE=IWE10+IWE22+IWE11. In other words, a period of a time difference between the first discharge start current IWE10 (2) and the final large electric discharge machining current IWE11 (9) is joined by the discharge maintenance current IWE22 (4) outputted from the power supply unit for preliminary discharge 102. Consequently, electric discharge machining can be performed repeatedly while a discharge state in the inter-electrode portion is maintained.
Here, when the FET 2 shown in FIG. 1 is used as the switching elements S11a and S11b, the delay time tr from the discharge start time t0 until the electric discharge machining current IWE11 appears in the inter-electrode portion is usually about 410 nanoseconds. In addition, a pulse width of capacitor discharge of the discharge start current IWE10 is about 360 nanoseconds. In 50 nanoseconds that is a difference between the delay time tr and the pulse width, although it is likely that discharge is cut off if this state is not changed, since the discharge maintenance current IWE22 flows as described above, the inter-electrode current IWE is never cut off.
However, in the conventional power supply device for electric discharge machining, an upper limit value of the discharge maintenance current IWE22 is limited by the resistor R21. In addition, a current value is reduced by the floating inductances L21 and L22 in the circuit in an initial stage of a transient state. Therefore, there is a problem in that the conductive path in the inter-electrode portion formed after occurrence of discharge cannot be maintained, and input of the electric discharge machining current IWE11 fails.
In particular, in a large wire electric discharge machining apparatus, since a distance between an inter-electrode portion and a power supply device in an electric discharge machining unit in the wire electric discharge machining apparatus is long, a power supply cable connecting the inter-electrode portion and the power supply deice is also long. As a result, a floating inductance in a circuit increases, and the discharge maintenance current IWE22 may not rise even after the discharge start current IWE10 disappears. Thus, a conductive path formed in the inter-electrode portion is cut off.
In addition, in the resistor R21, an inductance component due to a resistance winding is present, and an inductance also increases to obtain a necessary resistance. When the inductance of the resistor increases in this way, the rising of the discharge maintenance current IWE22 is further prevented. The first discharge start current IWE10 is a current due to capacitor discharge and actually includes an oscillation component. Therefore, even if a maximum value of the discharge maintenance current IWE22 is set somewhat large in advance, the discharge maintenance current IWE22 is offset by a component on a negative side of this oscillation, and the conductive path formed in the inter-electrode portion is cut off.
If the conductive path in the inter-electrode portion secured by the discharge start current IWE10 is cut off in this way before the electric discharge machining current IWE11 is inputted, an action of supplying the electric discharge machining current IWE11 stably using the discharge maintenance current IWE22 serving as a preliminary discharge current is not obtained. Thus, various failures occur in electric discharge machining.
In short, in a state in which the conductive path in the inter-electrode portion is cut off, since an output terminal of the power supply unit for machining discharge 101 is in an opened state, the electric discharge machining current IWE11 does not flow. Normal electric discharge machining is not performed in this case. When frequency of occurrence of such a state increases, the number of times of effective discharge decreases. As a result, there is a problem in that machining speed, which should be obtained originally, cannot be obtained, and further improvement of the machining speed cannot be realized.
Since a voltage at the DC power supply V11 is usually set two to three times as large as a voltage at the DC power supply V21 to output a large current in a short time. However, when there is no conductive path in the inter-electrode portion and the DC power supply V11 is in an opened state, a high voltage at this DC power supply V11 is applied to the inter-electrode portion. As a result, discharge is caused by this high voltage anew, and a large current flows to the inter-electrode portion suddenly without preliminary discharge. As a result, when a wire electrode is thin, the wire electrode is broken. Even if breakage does not occur in the wire electrode, there is a problem in that, for example, a machining surface is roughened, machining accuracy is deteriorated, and stable electric discharge machining characteristics cannot be obtained.
Concerning the problems as described above, Japanese Patent Application Publication No. H5-9209 (power supply for wire cut electric discharge machining apparatus) points out the same problems and, as a solution for the problems, discloses a technique for providing a circuit, in which an inductance and a capacitor are connected in series, in parallel with an inter-electrode portion and maintaining a conductive path of the inter-electrode portion after occurrence of discharge, that is, keeping a discharge state stably to prevent decline in machining efficiency. However, in this measure, since an excess capacitor is added in the inter-electrode portion eventually, for example, an electric capacitance viewed from a power supply device side increases according to a stray capacitance in the circuit, a rising time constant at the time when an output voltage is applied to the inter-electrode portion increases, and rising of an inter-electrode voltage is delayed. Consequently, since a voltage application time until discharge is caused becomes long, there is a disadvantage in that the number of times of effective discharge decreases and the machining efficiency cannot be improved sufficiently.
In addition, in the power supply for wire cut electric discharge machining apparatus disclosed in Japanese Patent Application Publication No. H5-9209, a peculiar oscillation frequency is obtained according to values of the inductance and the capacitor to be added. However, in recent years, a power supply device for electric discharge machining of a bipolar type, which alternately changes a polarity of a voltage applied to an inter-electrode portion to output oscillation, is mainly used. In this case, the added capacitor repeats charging and discharging operations according to at least an oscillation frequency of voltage application. A dielectric loss is present even in a capacitor for a high-frequency application. Therefore, in the technique disclosed in Japanese Patent Application Publication No. H5-9209, in addition to a problem of limiting the oscillation frequency of voltage application, there is also a problem in that heat generation occurs due to the dielectric loss, and a loss of supply energy is also caused.
The present invention realizes a large current and a high frequency necessary for high-speed machining such that both the large current and the high frequency can be attained. As prior examples concerning this point, for example, there are Japanese Patent Application Laid-Open No. H11-48039 (electric discharge machining power supply device for electric discharge machine), Japanese Patent Application Laid-Open No. S64-11713 (electric discharge machining power supply), and Japanese Patent Application Laid-Open No. H8-118147 (electric discharge machining power supply control device for wire electric discharge machine).
Japanese Patent Application Laid-Open No. H1-48039 (electric discharge machining power supply device for electric discharge machine) and Japanese Patent Application Laid-Open No. S64-11713 (electric discharge machining power supply) disclose a technique for supplying a large current to a machining gap but do not examine efficiency of a circuit and a heat loss. In other words, there is a problem in that, when a switching element excellent in a low-loss characteristic with a large current capacity is used to improve a heat loss and switching efficiency at the time when a large current is supplied, a gate input capacity is increased, a rising characteristic of a turn current is deteriorated, arc cut-off easily occurs, and wire breakage occurs frequently.
Japanese Patent Application Laid-Open No. H8-118147 (electric discharge machining power supply control device for wire electric discharge machine) discloses a technique for preventing wire breakage more surely by applying three kinds of large, medium, and small current pulses. However, since the power supply control device includes identical switching elements and drive circuits, for example, when a thick wire electrode of φ0.35 or the like is used, it is necessary to increase the number of parallels of switching elements. Thus, there is a problem in that cost increases and a power supply device cannot be reduced in size.
In short, in conventional examples including the prior examples described above, there is a problem in that, although a large current can be supplied, when a discharge frequency increases, an energy loss of a switching element increases rapidly, and the switching element is thermally broken. There is also a problem in that, even if the switching element is not thermally broken, it is necessary to excessively increase a capacity of a thermal converter to protect the switching element from an increased switching loss, and reduction in cost and reduction in size of a power supply device cannot be realized. For example, it is difficult to use a switching element, which has a low loss and is suitable for large current supply, like an IGBT in an area in which a repetition frequency is high (e.g., 40 kilohertz). Since, in general, a small current capacity switching element has a large ON resistance, a heat loss increases excessively when an ON time is long, and discharge treatment is costly.
The present invention has been devised in view of the problems described above, and it is an object of the present invention to obtain a power supply device for electric discharge machining in which switching circuits include two types of switching circuits with different characteristics, wire electric discharge machining coping with a large current and a high frequency can be performed efficiency with the switching circuit, and the number of switching elements and a heat value of the switching elements can be reduced.